In a taped interview, Robert Emerson Robinson, 28, admitted to having sex with the unnamed teen girl while he was working at the Harris County Juvenile Justice Center.

According to the criminal complaint, the May 23 incident was revealed after the teenager came forward, saying she allowed Robinson to have sex with her "because he had provided her with candy and treats while she was incarcerated."

Surveillance footage on the fourth floor of the detention center from May 23 shows Robinson entering a cell housing the teenager. Robinson entered the girl's cell during a time when the lights were off. In the footage, only Robinson's arm is exposed in the doorway, according to the complaint.

Robinson told investigators that he "made a mistake," and that the teen was telling the truth about the sexual activity. He has been charged with sexual assault of a child under age 17.

It is unclear if he has been fired from his job. Calls placed by ABC News to Kendall Mayfield with the Harris County Juvenile Probation Department were unreturned.

Robinson was released from custody at the Harris County Jail after posting $30,000 bond, court records indicate.

The details of this incident at the Juvenile Justice Center emerging this week come as a second blow to Harris County following the firing last week of six employees in connection to alleged sex scandal at other county jails.

Attorney Amin Alehashem, director of the Texas Civil Rights Project, a non-profit legal organization focusing on indigent residents, civil rights, and prisoners rights, told ABC News that he is "sadly not surprised" by what he calls these "horrible and vile" acts.

"The issue in general raises questions, and I'd like to know how an officer went into a cell unnoticed, on at least one occasion. How is it that nobody knew? And how is it that it wasn't stopped?" he said.

On Friday, Sheriff Adrian Garcia announced that after an investigation that was launched in August of last year, a deputy, a supervisor, and four civilian detentions officers – some of whom had been employed for 10 years -- had been fired for violating the misconduct at two jail facilities in downtown Houston. It was discovered that the employees were engaging in sex acts with inmates and with other staff at the jail, primarily in the laundry rooms.

"Their actions are inexcusable because it was crystal clear to all of our employees that this kind of conduct is wrong," Sheriff Garcia said in a statement. "The laws of Texas, the rules of the Sheriff's Office, our training programs and common sense all say so."

During the investigation, one deputy and one supervisor resigned. Some were accused of having sex with inmates or colleagues, while others were supervisors accused of looking the other way. Harris County Deputy Tony Richards was fired in February and has been charged with having improper sexual activity with an inmate.

Alan Bernstein, a spokesman for the Harris County sheriff's office, told ABC News that the investigation was launched when a sheriff became suspicious of what he was seeing in the jails.

"It started not with anyone coming forward, but with a sheriff noticing a female inmate with new tennis shoes, and wondering where they came from. It turned out it was a favor done my employee," he said. "The ball of yarn unraveled from that point."

Bernstein said that many of the facilities in Harris County are buildings retrofitted to be jails, and that the original design did not include cameras or clean sight lines. He said following the investigation, they plan to add more surveillance cameras and reexamine their training process.

"We did provide training to all jail employees of common sense rules, and criminal law of the state of Texas," he said. "They're told it's a felony. We've told people that before, and that it's a violation of our internal rules."

Alehashem said that though this is a potential start, the full extent of what's going on in Harris County's jail and Juvenile Justice Center has yet to be properly diagnosed.

"If these were all taking place in the laundry room, it sounds like there was a culture in the jail where there was a place to do this sort of thing," he said. "We need to look at how this culture started, who else knew about this, and look at every case and see what the common links are and how this is allowed to go on."